is it still under warranty and could you take it back to your dealer, what type of decoder are we talking about ? 21 mtc pin or an 8 pin ?
you could go to a Märklin dealer and check if the decoder is faulty before going down the track and buying a new one

John

Are you trying to use the train on an analog or digital layout?

Pay attention to the position of the dip switches as mentioned in the instructions:

"Manually setting the operating mode.
The digital address is set to “03” ex works. In order to operate this version with alternating current, the settings at the switch groups on the decoder must first be changed.
In this case, move all switches in both switch groups to the “off” position.
Note: As a result, this train cannot switch from one system area with a digital power supply to another system area with conventional alternating current."

Both versions of this train cannot be operated with other makes of locomotive controllers
(DC power, pulse width control, other makes of multi-train systems).

"Safety Warnings
This locomotive is to be used only with an operating system designed for it (Märklin 6646/6647
AC transformer, Märklin Delta or Märklin Digital).
This locomotive must never be supplied with power from more than one transformer."

I do not know what happens if the model is used on a system with those switches not correctly set.

Might I also inquire as to which controller you are using (6021, MS, CS)?
The instructions might suggest that there may be some issue when used with the MS/CS with DC power.

As far as lights, it says that the LD lights will only work if the 6021 has dip switch 2 set to "on"

Right Click and select "Open in New Window": https://static.maerklin....f6ecf2c4921434542204.pdf
Apple. Two fingers on the trackpad and select "Open in New Window"

Regards

Mike C

When you buy a 20-year-old shelf-queen from an authorised dealer, you will have 2 years of warranty.
But odds to get spare parts would still be not good.

In this case it seems the train is out of warranty.